1. Field of the Invention
This invention relates generally to wireline blowout preventers (BOP""s) which are utilized to prevent well blowout when wireline operations are being conducted. More specifically, the present invention concerns wireline blowout preventers which incorporate pressure responsive sealing rams each having a one-piece ram element block for sup port and positioning of an elastomer sealing element for sealing with a wireline and for preventing misalignment of seal support plates especially when the plates are machined to accommodate larger wire sizes.
2. Description of the Prior Art
After a well has been drilled such as for discovery and production of petroleum products, wireline controlled apparatus is frequently employed to conduct various downhole installation, retrieval and servicing operations. Wireline equipment is utilized to install and retrieve a wide variety of downhole tools such as packers, gas lift valves, downhole safety valves, bottom hole pressure sensors and the like. Wireline equipment is also frequently utilized to run various well servicing tools such as for cleaning and treating production tubing. The wireline for supporting and controlling movement of the downhole well tool is payed out from a large diameter spool, typically mounted to a service vehicle, and is run through an elongate lubricator body having an internal grease or lubricant chamber containing grease for maintaining a seal between the wireline and the lubricator body to prevent escape of well pressure as the wireline is moved linearly into and from the well.
At times it is desirable to establish a seal about the wireline to permit well servicing operations to be conducted within the well head and free of well pressure above the blowout preventer (BOP). More importantly, wireline BOP""s are required to accommodate various emergency situations and alleviate any dangerous condition that might otherwise occur. During wireline operations the wireline BOP is typically static in an open, non-sealing position which allows the wireline to freely traverse the wellhead of the well without interference with any portion of the wellhead structure including the safety equipment of the wellhead.
At times during well servicing operations it will be necessary to quickly close and seal the wireline BOP about the wireline. For example, in the event a kick develops in the open hole or in the casing of the well, the wireline BOP is closed to develop a seal between the wireline and the BOP body to contain well pressure and prevent a blowout. Such emergency closure and sealing of a BOP can be manually accomplished or it can be automatically operated responsive to the well pressure condition. With well pressure thus contained, kill fluid can be pumped into the well below the seal established by the wireline BOP to counterbalance the well pressure condition and shut in the well. The wireline BOP may also be closed to temporarily maintain well pressure while bleeding off the lubricator of wireline equipment to replace packoff elements. In some cases it becomes necessary to remove, add or make repairs to the riser or cable of the wireline well servicing equipment. In this case the wireline BOP is typically closed and sealed about the wireline cable to thus permit bleeding of pressure from the lubricator riser, enabling cable repairs to be made above the level of the wireline seal within the BOP. Most critically, wireline BOP""s are shut automatically or manually in the case of failure of the well control system above the level of the BOP.
Typically, wireline sealing elements are formed by two elastomer faced metal rams which have been contoured to fit a particular size of wireline cable. These rams are positioned in opposed relation and are actuated such that they establish sealing engagement with the wireline positioned centrally of the BOP housing. In other words, the opposed rams center the wireline within the housing and establish a seal about the wireline. The opposed rams are closed manually or hydraulically (with mechanical backup) to shut in the well. When the BOP is activated, the elastomer faces of the rams will seal around the outer surface of the cable by extruding the elastomeric material into the interstitial spaces between the armor strands of the wireline.
The ram elements each define a ram body that is linearly movable within a transversely oriented ram passage or chamber of a BOP body. The ram element include at least one external seal which establishes sealing of the ram body with respect to the internal surface of the ram passage or chamber. At the opposed sealing face of each ram body, the ram body defines at least one and preferably a pair of seal receptacles. Within each seal receptacle is located a seal body, typically comprising an elastomeric sealing body having metal support plate members providing upstream and downstream structural support for the elastomeric sealing body and providing the elastomeric sealing body with sufficient structural integrity to resist the force of pressure differential that might otherwise permit sufficient yielding of the elastomeric sealing body to allow leakage to occur. The metal support plate members also provide the elastomeric sealing body with enhanced structural integrity to resist excessive pressure induced extrusion of the elastomeric material especially in the region of its seal with the wireline. Typically the elastomeric body and the metal support plates are in molded assembly and thus form a ram seal unit than can be replaced when necessary without necessitating replacement of the ram body.
When the ram seal units are manufactured by a molding operation, the metal support members are located within a molding fixture and the elastomeric material is injected in uncured state into the molding fixture and then cured by heating to a suitable temperature range for a predetermined period of time. Since the uncured elastomeric material is quite viscous during molding, the metal support places can be slightly displaced, i.e., moved from side to side, during elastomer injection especially when the support plates are machined to allow for larger wireline size. When the support plates become misaligned during the molding operation, the misaligned wireline recesses of the plates will be misaligned and the resulting wireline channel of the ram seal unit will be improperly constructed. This manufacturing problem can cause the sealing integrity of the ram seal unit to be of less than desirable quality. Additionally, during the molding operation the support plates can become bowed, thus also adversely influencing the sealing capability of the ram seal unit. Another problem that is often encountered during the seal manufacturing process is that the upper and lower support plates can easily become improperly spaced. It is necessary the support plates be disposed in parallel relation and that the spacing of the support plates of each of the ram seals of each of the BOP rams have the same spacing to enable equal elastomer deformation in each of the ram seals in response to a particular pressure differential. It is desirable therefore to provide a ram seal unit that can be easily manufactured in a manner avoiding the above-identified problems.
Ram type BOPS are designed to seal with the wireline cable in a static position. It is necessary therefore to always stop movement of the wireline before the rams are closed about the wireline to effect sealing. Short lengths of wireline cable can be stripped through the BOP as needed to repair a stranded cable but the amount of elastomeric material that can be worn away by such stripping without resulting in leakage of the BOP is limited. Pulling an appreciable length of wireline cable through the BOP will usually induce severe wear or erosion of the elastomeric seals of the rams, resulting in leakage, and can cause damage to the ram s as well, causing a more serious failure.
Under conditions of high differential pressure the elastomeric seal is prevented from pressure induced extrusion by the close metal-to-metal fit of the ram faces and due to the fact that differential pressure moves the cylindrical rams tightly against the upper wall of the BOP housing. When the mechanical pressure of the elastomer against the surrounding surfaces of the BOP body exceed differential pressure across the sealing body a leak-tight seal will be effected. Wireline BOPs are typically designed to seal against well pressure in only one direction and therefore care must be taken to insure that they are not installed upside-down when a single set of rams is employed because in most cases an inverted BOP will not hold well pressure. BOP installations having single ram BOPs are usually only installed upside-down to contain pressure injected from above. BOP units incorporating a centralized grease pack feature between the upstream and downstream ram seal units can have pressure sealing capability both from the standpoint of containing well pressure and containing pressure injected above the BOP seals. In this case the BOP units are not unidirectional and may be installed in an orientation suiting the needs of the user.
It is difficult to move the rams of wireline BOPs when the rams are closed against high differential pressures. Wireline BOPs are therefore typically provided with a pressure equalization system, typically referred to as a bypass, that is used to equalize the pressure across the rams before opening of the rams is initiated. After equalization of well pressure across the rams has occurred, there will be a partial relaxation of the sealing contact of the elastomeric seals with the wireline and with the BOP housing, thus reducing extrusion of the elastomeric sealing material about the wireline and minimizing friction that otherwise tends to resist opening movement of the rams. The effective reduction of friction resulting from pressure equalization across the rams makes the rams much easier to open.
Manual BOPs are operated by turning two ram operator handles on opposed sides of the apparatus to open and close the opposed rams. Manual BOP""s are available in a number of sizes and ratings. Regardless of the pressure rating, they are normally used at lower pressures for standard service. Manual BOPs are typically of lighter weight and are less expensive as compared to BOPs having hydraulically energized rams. Because operating personnel must gain physical access to manual BOPs for opening and closing the rams, such personnel is typically in a more dangerous location during these activities. Also, less ram pressure can be applied with manual BOPs than with hydraulically energized ram BOPs. In larger sizes, as commonly used for open hole work, the manual BOP offers adequate protection and is considerably lighter than a hydraulically energized BOP.
Hydraulic ram BOPs are opened and closed by hydraulic pressure acting on pistons in hydraulic cylinders. Hydraulic BOPs also have handles and stems that are used for manual backup. A hydraulic BOP can be closed manually but must be opened hydraulically. The stems must be backed out manually before the rams can be opened hydraulically.
The rams and sealing elements of wireline BOPs have grooves that are sized for the wireline cable diameter being employed. As the rams are, closed, the cable is guided by the rams or other cable guide elements into the grooves of the sealing elements. BOPs are provided with xe2x80x9cintegral guidexe2x80x9d rams that prevent cable damage as the cable is guided and centralized during ram closure. If the rams of hydraulic wireline BOPs are to be left closed for a long period of time or in case of hydraulic failure, the manual screw jack can be used to hold the rams in the closed position. To open a hydraulic wireline BOP, the mechanical backup must be in the open position before shifting the xe2x80x9cselectorxe2x80x9d to its open position and hydraulically pumping the rams to their open positions.
Multiple ram BOPs, typically dual wireline BOPs, are utilized to provide a backup in case of failure of the primary set of sealing rams. More importantly is the fact that gas will tend to migrate through the interstices between the inner and outer armor strands of a stranded type wireline cable. In the event that the lubricator of the wireline equipment would need to be removed for some reason, the leaking gas could quickly present a significant problem from the standpoint of danger to personnel. To alleviate this problem, a second (tandem) BOP is added. This second BOP is inverted if it is designed to hold pressure in only one direction and a port is added between the two BOP""s. High pressure grease (above well pressure) is injected into the flow passage between the upper and lower sets of rams. Under high pressure, the grease is caused to migrate into the interstitial spaces between the inner and outer armor strands of the wireline and thereby effects an interstitial grease seal to prevent gases from escaping. In some cases a triple BOP is also available which provides a backup in the case of primary ram failure. A triple BOP also provides a method for injection of grease between the BOPs if needed. Multiple ram BOPs are available in a single forged body for lighter weight and more compact size.
As explained, it is frequently necessary to employ dual and sometimes triple wireline BOPs and to provide for grease injection between them in order to effect a proper wireline seal and prevent migration of gases through the interstices of the wireline. Since the flow passage between the stacked or multiple wireline BOPs is typically of significant length and is at least as great as the diameter of the flow passage through the wireline, a considerable volume of injected grease is necessary to accomplish efficient sealing. Also, injection of a sufficient volume of grease to fill the flow passage and develop a hydraulic seal with the wireline can require a considerable period of time. It is desirable to minimize the time required to develop an adequate hydraulic seal with the wireline to thus promote the safety of the sealing operation.
It is a principal feature of the present invention to provide a novel sealing ram construction for blowout preventers, particularly wireline blowout preventers which incorporates an integral ram seal block to which elastomer sealing material is molded and which integral ram seal block provides integral support plate structures for resisting elastomer extrusion when the BOP rams are closed and subjected to significant pressure differential;
It is another feature of the present invention to provide a novel sealing ram construction for blowout preventers which eliminates the potential for support plate twisting, misalignment, lateral shifting, etc. during manufacture of ram seal units for attachment to the rams of blowout preventers;
It is also a feature of the present invention to provide a novel sealing ram construction for blowout preventers which simplifies and minimizes the cost of manufacture of ram seal units for blowout preventers without sacrificing the quality thereof;
Briefly, ram seal units for attachment to the rams of blowout preventers, manufactured within the spirit and scope of the present invention, incorporate an integral ram seal block having a central web structure, with a pair of generally rectangular elastomer support plates extending from opposite sides of the integral ram seal block and being disposed in parallel relation. The central web structure is perforate to permit its interlocked relation with elastomer sealing material that is molded to ram seal block The generally rectangular elastomer support plates define outer face surfaces that remain exposed after the molding operation has been completed. These outer face surfaces of the elastomer support plates are adapted to be disposed in intimate face-to-face relation with internal wall surfaces of ram seal receptacles that are provided within BOP ram elements. Each of the rectangular elastomer support plates defines a pair of spaced retainer receptacles which receive a respective retainer element of a BOP ram to secure the ram seal units in retained relation within one of the ram seal receptacles of a BOP ram element. When so supported, the elastomer sealing material located between the generally rectangular elastomer support plates and encapsulating the central web structure of each integral ram seal block will be presented for sealing contact with its opposed counterpart of the opposed BOP ram element and with the wireline that is present within the body of the BOP unit.
Each of the plurality of ram seal units of opposed BOP rams will be adapted for the size of the wireline for which the BOP unit is designed. After the molding operation has been completed the ram seal units will be machined to define a wireline groove of proper size to fit the wireline. Alternatively, before the molding operation is initiated, the integral metal ram seal block can be machined to define a wireline groove of proper size and centralized location in the spaced support plate structures thereof. The elastomer molding operation can then be carried out to form a wireline groove in the elastomer material which may be of different dimension as compared with the wireline groove in the support plates to accommodate displacement of the elastomer sealing material by the mechanical force holding the sealing faces of the rams in sealing contact. As a further alternative, subsequent to the molding operation, the elastomer material can be separately machined in the region of the wireline groove to form a wireline groove in the elastomer material which can be of different dimension as compared with the wireline groove machined in the support plates of the integral metal ram seal block.
After the elastomer sealing material has been molded to each of the integral ram seal blocks and the wireline groove has been formed in each of the ram seal units, the ram seal units are then installed within respective receptacles formed in the sealing face of the BOP ram elements. Typically, each of the BOP ram elements will define a pair of spaced ram seal receptacles which are disposed in spaced relation. The space between the ram seal units can be employed to define a small volume grease receptacle, such as the case when the opposed BOP rams and ram seals form a grease pack BOP sealing system of the general nature disclosed in U.S. Pat. No. 4,93 8,290 of Henry H. Leggett, et al.
Since the elastomer support plates are integrally formed with the integral ram seal block, plate misalignment, bending, twisting or shifting during the molding process will not occur. The plate structures will remain precisely parallel during the molding operation so that the outer plate surfaces will precisely fit in face-to-face relation with the internal planar surfaces of the ram seal receptacles of the BOP ram elements.